How to query data using primary key in MySQL

MySQL is a widely used open source relational database that supports rich query and operation functions, and the most commonly used query method is to query data through primary keys. This article will introduce how to use primary keys to query data in MySQL, including the definition, use, query, optimization, etc. of primary keys.

1. Definition of primary key
The primary key is the unique identifier used to identify each record in the table in MySQL. The primary key can be composed of a single field or a combination of multiple fields, but it must meet the following conditions:

1. Uniqueness: The primary key must be unique and cannot duplicate existing data in the table.
2. Non-nullability: The value of the primary key cannot be NULL.
3. Stability: The value of the primary key cannot change with time or other conditions.

When defining the primary key, you can use MySQL's auto-increment function. For example:

CREATE TABLE user (
id int(11) NOT NULL AUTO_INCREMENT,
name varchar(50) NOT NULL,
password varchar(50) NOT NULL,
email varchar(50) NOT NULL,
PRIMARY KEY (id )
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

In the above code, the id field is defined as the primary key, and the auto-increment function of MySQL is used. When new data is inserted, the value of the id field is automatically incremented by 1 to ensure that the primary key value of each record is unique.

2. Use of primary key

1. Insert data
   When inserting data into the table, the value of the primary key must be specified. If you use the auto-increment function, there is no need to specify a primary key value. For example:

INSERT INTO user (name, password, email) VALUES ('Tom' , '123456', 'tom@example.com');

2. Update data
   When updating data, you need to specify the primary key value of the record to be updated. For example:

UPDATE user SET password='654321' WHERE id=1;

3. Delete data
   When deleting data, you also need to specify the primary key value of the record to be deleted. For example:

DELETE FROM user WHERE id=1;

3. Primary key query
Using the primary key to query data is The most common and basic query method in MySQL. When performing a primary key query, you can use the following two methods:

1. Use the WHERE clause
   Specify the primary key value through the WHERE clause to query, for example:

SELECT * FROM user WHERE id=1;

2. Use primary key query syntax
   Use primary key query syntax directly, for example:

SELECT * FROM user WHERE id=1;

4. Primary key index
   In MySQL, when querying data using the primary key, The index of the primary key plays a vital role. Indexes can greatly improve query efficiency and return query results faster. Therefore, when designing a database table, the primary key should be indexed according to actual needs. The primary key can be indexed using the following methods:
5. Create the index at the same time when creating the primary key. For example:

CREATE TABLE user (
id int(11) NOT NULL AUTO_INCREMENT,
name varchar (50) NOT NULL,
password varchar(50) NOT NULL,
email varchar(50) NOT NULL,
PRIMARY KEY (id ),
INDEX idx_user_name (name)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

In the above code , by creating an index on the name field, you can improve the query speed when querying the name field.

2. ALTER TABLE adds index. For example:

ALTER TABLE user ADD INDEX idx_user_email (email);

In the above code, Added index for email field.

Summary
This article introduces the definition, use and query method of primary keys in MySQL. In practical applications, primary keys should be optimized and indexed according to actual needs to improve query efficiency and performance.

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